This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 is a view showing an example of a conventional III-nitride semiconductor light-emitting device. The III-nitride semiconductor light-emitting device includes a substrate 10 (e.g., sapphire substrate), a buffer layer 20 grown on the substrate 10, an n-type III-nitride semiconductor layer 30 grown on the buffer layer 20, an active layer 40 grown on the n-type III-nitride semiconductor layer 30, a p-type III-nitride semiconductor layer 50 grown on the active layer 40, a current spreading conductive film 60 formed on the p-type III-nitride semiconductor layer 50, a p-side pad electrode 70 formed on the current spreading conductive film 60, an n-side pad electrode 80 formed on the n-type III-nitride semiconductor layer 30 exposed by mesa-etching the p-type III-nitride semiconductor layer 50 and the active layer 40, and a protective film 90.
The current spreading conductive film 60 serves to allow current to be smoothly supplied to the entire p-type III-nitride semiconductor layer 50. The current spreading conductive film 60 may be provided almost on the entire surface of the p-type III-nitride semiconductor layer 50 and formed as a light-transmitting conductive film using ITO or Ni and Au or as a reflective conductive film using Ag, for example.
The p-side pad electrode 70 and the n-side pad electrode 80, which are metal electrodes for current supply and external wire bonding, may be made of any one selected from the group consisting of Ni, Au, Ag, Cr, Ti, Pt, Pd, Rh, Ir, Al, Sn, In, Ta, Cu, Co, Fe, Ru, Zr, W, and Mo, or a combination thereof, for example.
The protective film 90, which is made of SiO2, can be omitted.
With larger areas and higher power consumption of the semiconductor light-emitting device, a plurality of pad electrodes and branch electrodes have been introduced for smooth current spreading in the semiconductor light-emitting device. For instance, FIG. 2 is a view showing an example of an electrode structure described in U.S. Pat. No. 6,307,218. Here, as the semiconductor light-emitting device becomes larger, branch electrodes 910 are disposed between a p-side pad electrode 710 and an n-side pad electrodes 810 at regular intervals so as to improve current spreading.
Although the light-transmitting conductive film and the branch electrode are electrically connected to each other, there is a tendency that current is concentrated on the p-side pad electrode, the n-side pad electrode, and the branch electrode. When current is concentrated on local regions of the semiconductor light-emitting device, light cannot be uniformly emitted from the entire light emission area of the semiconductor light-emitting device, which degrades characteristics of the semiconductor light-emitting device in the long term.
In order to solve the foregoing problem, an insulator is provided as a current blocking layer (CBL) under the pad electrode and the branch electrode so that current can be better spread to the entire light-transmitting conductive film. However, the refractive index (e.g., 1.8-2.0) of the light-transmitting conductive film, which is made of ITO, etc., is greater than the refractive index (e.g., 1.5) of the insulator, which is made of SiO2, etc. As such, light is refracted on the interface between the insulator and the light-transmitting conductive film, and the reflection factor of light is reduced on the pad electrode and the branch electrode. As a result, there is a problem that light extraction efficiency of the semiconductor light-emitting device is reduced.